Oil well logging has been known for many years and provides an oil and gas well driller with information about the particular earth formation being drilled. In one type of oil well logging, after a well has been drilled, a probe, or sonde is lowered into the borehole to measure certain characteristics of the formations through which the well has passed. The probe hangs on the end of a cable which gives mechanical support to the sonde and which provides power to the sonde. The cable also conducts information up to the surface. Such "wireline" measurements are made after the drilling has taken place.
A wireline sonde usually contains a source which transmits energy into the formation as well as a suitable receiver for detecting energy returning from the formation. The energy can be nuclear, electrical, or acoustic. Wireline "gamma-gamma" probes, for measuring formation density, are well known devices incorporating a gamma ray source and a gamma ray detector. During operation of the probe, gamma rays emitted from the source enter the formation to be studied, and interact with the atomic electrons of the material of the formation by the photoelectric absorption, by Compton scattering, or by pair production. In photoelectric absorption and pair production phenomena, the particular gamma rays involved in the interaction are consumed in the process.
In the Compton scattering process, the involved gamma ray loses some of its energy and changes its original direction of travel, the amount of energy loss being related to the amount of change in direction. Some of the gamma rays emitted from the source into the formation are scattered by this process toward the detector. Many of these rays fail to reach the detector, since their direction is again changed by a second Compton scattering, or they are absorbed by the photoelectric absorption process or the pair production process. The scattered gamma rays that ultimately reach the detector and interact with it are counted by the electronic circuitry associated with the detector.
Wireline formation evaluation tools such as the aforementioned gamma ray density tools have many drawbacks and disadvantages, including loss of drilling time and the expense involved in pulling the drillstring so as to enable the wireline to be lowered into the borehole. In addition, a substantial mud cake can build up, and the formation can be invaded by drilling fluids during the time period that drilling is suspended. An improvement over these wireline techniques is the technique of measurement-while-drilling (MWD), which measures many of the characteristics of the formation during the drilling of the borehole. Measurement-while-drilling can totally eliminate the necessity for interrupting the drilling operation to remove the drillstring from the borehole. The present invention relates to a measurement-while-drilling apparatus. Specifically, this invention is most useful in such an instrument which measures the density of the formation wherein the source emits gamma rays.
In a typical MWD density tool, an instrument housing, such as a drill collar, is provided which incorporates a single gamma ray source and a pair of longitudinally displaced and mutually aligned detector assemblies. A nuclear source is mounted in a pocket in the drill collar wall and partially surrounded by gamma ray shielding. The two detector assemblies are mounted within a cavity or hatch formed in the drill collar wall and enclosed by a detector hatch cover under ambient pressure. The detector assemblies are spaced from the source and partially surrounded by gamma ray shielding to provide accurate response from the formation. The hatch cover contains radiation transparent windows in alignment with the detector assemblies.
The density instrument housing may include a central bore for internal flow of drilling fluid. The drill collar wall section adjacent to the source can be expanded radially so as to define a lobe which essentially occupies the annulus between the drill collar and the borehole wall. A radiation transparent window is provided in the lobe to allow gamma rays to reach the formation, and the surrounding lobe material reduces the propagation of gamma rays into the annulus. Reduction of the gamma ray flux down the annulus is desirable to reduce the number of gamma rays which reach the detector through the drilling fluid without passing through the formation.
Another means frequently used to reduce the gamma ray flux through the drilling fluid to the detectors is a threaded-on fluid displacement sleeve positioned on the drill collar and over the detector hatch cover. Examples of such a sleeve can be found in U.S. Pat. Nos. 5,091,644 and 5,134,285. In lieu of the lobe around the source port described above, the fluid displacement sleeve can extend over the source port as well as the detector ports. This sleeve displaces borehole fluids as mentioned above, reduces mudcaking which might have an adverse effect on the measurement, and maintains a relatively constant distance between the formation and the detector. The sleeve typically used has blades which are full gage diameter, matching the borehole diameter, or they can be slightly under gage, and adequate flow area for drilling fluids is provided between the blades. One blade is positioned between the detectors and the borehole wall to displace fluid from the annular space between the detectors and the formation. The other blades are positioning blades which position the instrument centrally within the borehole and which hold the fluid displacement blade against the formation. The blades are hard faced with wear resistant material. The threading and shoulders of the sleeves are configured so as to adequately secure the sleeve to the drill collar without rotation while drilling. The sleeve may be replaced at the drilling site when worn or damaged.
The problem with MWD instruments of this type is that a different instrument is required for each diameter of borehole. Detector to formation distance is critical, and drilling fluid must be displaced from the annular space between the detector and the borehole wall. Therefore, each borehole diameter requires the design and manufacture of an instrument, instrument housing, and fluid displacement sleeve specifically intended for use only in a borehole of the given diameter. Not only is design and manufacture of a full range of tools expensive, but each tool must be extensively modeled and mathematically calibrated for use in the given diameter of borehole, and acceptance testing must be performed on each different design. Even if a single instrument were used, with different diameters of fluid displacement sleeves, calibration and modeling effort would be necessary for each sleeve design. Further, the use of a different tool in each diameter of borehole requires a logging company to maintain a large inventory of tools, along with the associated difficulty in handling, storing, and testing such tools.
There is a continuing need, therefore, for an improved MWD density tool in which a single design instrument can be used in a variety of different sizes of boreholes without the need for recalibration, computer modeling, or repeated acceptance testing. Specifically, improvements are possible in achieving accurate and reliable measurements, with a single instrument, in different size boreholes, while minimizing the presence of drilling fluid between the tool's nuclear detectors and the formation.